The present invention generally relates to definition of an image field of view. In particular, the present invention relates to a system and method for interactive definition of an image field of view in digital radiography.
Digital imaging systems may be used to capture images to assist a physician in making an accurate diagnosis. Digital radiography imaging systems typically include a source and a detector. Energy, such as x-rays, produced by the source travel through an object to be imaged and are detected by the detector. An associated control system obtains image data from the detector and prepares a corresponding diagnostic image on a display.
One or more collimators may be used to block and/or restrict x-rays or other energy directed at a detector. For example, collimator blades may be used to form an opening through which x-rays pass from a source to a detector. An example of sizing and aligning collimator blades may be found in U.S. Pat. No. 6,215,853, entitled “Apparatus and Method for X-ray Collimator Sizing and Alignment”, which is herein incorporated by reference in its entirety.
The detector may be an amorphous silicon flat panel detector, for example. Amorphous silicon is a type of silicon that is not crystalline in structure. Image pixels are formed from amorphous silicon photodiodes connected to switches on the flat panel. A scintillator is placed in front of the flat panel detector. For example, the scintillator receives x-rays from an x-ray source and emits light of an intensity related to the amount of x-rays absorbed. The light activates the photodiodes in the amorphous silicon flat panel detector. Readout electronics obtain pixel data from the photodiodes through data lines (columns) and scan lines (rows). Images may be formed from the pixel data. Images may be displayed in real time. Flat panel detectors may offer more detailed images than image intensifiers. Flat panel detectors may allow faster image acquisition than image intensifiers depending upon image resolution.
A solid state flat panel detector typically includes an array of picture elements (pixels) composed of Field Effect Transistors (FETs) and photodiodes. The FETs serve as switches, and the photodiodes are light detectors and image storage elements. The array of FETs and photodiodes may be composed of amorphous silicon. A compound such as Cesium Iodide (CsI) is deposited over the amorphous silicon. CsI absorbs x-rays and converts the x-rays to light. The light is then detected and stored by the photodiodes. The photodiode acts as a capacitor and stores the charge.
Initialization of the detector occurs prior to an exposure. During an initialization of the detector, the detector is “scrubbed” prior to an exposure. During scrubbing, each photodiode is charged to a known bias voltage that represents “black”, or no light output. The detector is then exposed to x-rays which are absorbed by the CsI deposited on the detector. Light that is emitted by the CsI in proportion to x-ray flux causes the affected photodiodes to conduct, partially discharging the photodiode. After the conclusion of the x-ray exposure, the voltage on each photodiode is gated through a FET switch to an analog voltage comparator, which compares the photodiode's stored voltage with the voltage generated from a digital to analog (D/A) converter. The digital input to the D/A converter begins at ‘0’ and is incremented through a programmable ramp to a maximum value. As the analog ramp increases on the output of the D/A converter, the output eventually equals or exceeds the voltage coming from the photodiode, at which time the analog voltage comparator latches the current value of the D/A converter, which represents the digital pixel value for that photodiode.
The detector is read or scrubbed according to the array structure. That is, the detector is read on a scan line by scan line basis. A FET switch associated with each photodiode is used to control reading of photodiodes on a given scan line. Reading is performed whenever an image produced by the detector includes data, such as exposure data and/or offset data. Scrubbing occurs when data is to be discarded from the detector rather than stored or used to generate an image. Scrubbing is performed to maintain proper bias on the photodiodes during idle periods. Scrubbing may also be used to reduce effects such as incomplete charge restoration of the photodiodes, for example. Scrubbing restores charge to the photodiodes but the charge may not be measured. If the data is measured during scrubbing, the data may simply be discarded.
Switching elements in a solid state detector minimize a number of electrical contacts made to the detector. If no switching elements are present, at least one contact for each pixel is present on the detector. Lack of switching elements may make the production of complex detectors prohibitive. Switching elements reduce the number of contacts to no more than the number of pixels along the perimeter of the detector array. The pixels in the interior of the array are “ganged” together along each axis of the detector array. An entire row of the array is controlled simultaneously when the scan line attached to the gates of the FETs of pixels on that row is activated. Each of the pixels in the row is connected to a separate data line through a switch. The switch is used by read out electronics to restore charge to the photodiode. As each row is activated, all of the pixels in the row have the charge restored to the respective photodiodes simultaneously by the read out electronics over the individual data lines. Each data line typically has a dedicated read out channel associated with the data line.
Additionally, the detector electronics may be constructed in basic building blocks to provide modularity and ease of reconfiguration. Scan drivers, for example, may be modularized into a small assembly that incorporates drivers for 256 scan lines, for example. The read out channels may be modularized into a small assembly that would read and convert the signals from, for example, 256 data lines. The size, shape, architecture and pixel size of various solid state detectors applied to various imaging systems determine the arrangement and number of scan modules and data modules to be used.
A control board is used to read the detector. Programmable firmware may be used to adapt programmable control features of the control board for a particular detector. Additionally, a reference and regulation board (RRB) may be used with a detector to generate noise-sensitive supply and reference voltages (including a dynamic conversion reference) used by the scan and data modules to read data. The RRB also distributes control signals generated by the control board to the modules and collects data returned by the data modules. Typically, the RRB is designed specifically for a particular detector. An interface between the control board and the RRB may be implemented as a standard interface such that signals to different detectors are in a similar format.
Three-dimensional (3D) volumetric imaging (example shown in FIG. 1) provides new diagnostic and clinical analysis tools to physicians. 3D images are created by acquiring a series of two-dimensional (2D) images at predetermined positions along an arc about a patient. Software applications using complex mathematical processes extract volume elements or “voxels” from the 2D images by using the image content (e.g., a black-and-white x-ray image) and positional information (e.g., where the image was positioned along an arc). The voxels may then be assembled into a three-dimensional image and then viewed from any angle.
Due to the complex mathematics involved, it is important that the x-ray source be as directly centered above the x-ray detector and that the detector be as precisely perpendicular in both the X and Y planes of the beam as possible. The positional tolerances for mechanical mounting are typically small, in the range of ±0.5 mm (about twelve-thousandths of an inch).
Additionally, many imaging products are mobile, which offers hospitals, clinics, and physicians the ability to move these systems from room-to-room or to bring x-ray capability to a patient that cannot be moved. With the benefit of mobility also comes the risk of collision. Even if the systems are stationary, an accidental collision with a patient or operator may shift the detector. Due to the extremely tight tolerances required for 3D volumetric imaging, even small collisions may cause shifts in the x-ray detector and result in the degradation or deformation of the resulting 3D image. Traditional camera-based detector systems do not provide positional locating ability and may operate with a drift of several picture elements or “pixels” in position. Because these shifts may be imperceptible to the human eye, it would be highly desirable to know if the x-ray detector has shifted, and very advantageous to be able to automatically correct for any positional errors that may occur.
Furthermore, the Food and Drug Administration (FDA) places limits on radiation exposure to patients. The FDA is concerned with radiating a patient and being unable to use the resulting image due to calibration errors in the imaging system. Thus, a system and method which reduce a number of unusable images would be highly desirable.
Therefore, there is a need for an improved method and system for detecting alignment errors in imaging systems. Further, there is a need for an improved method and system for correcting alignment errors in imaging systems.